FRANKFURT — The disclosure that a botched plot to bomb two German trains last month involved a 21-year-old Lebanese man has shifted the spotlight of Islamic terrorism from Britain to Germany, rattling a country that has so far avoided such attacks on its own soil.

The German police have mounted an intense manhunt for a second suspect in the plot, having arrested a man they identified only as Youssef Mohamed E.H. on Saturday in the northern city of Kiel.

Prosecutors said the two men left suitcases stuffed with crude propane- gas bombs on the trains. The explosives failed to detonate because of a "technical defect," according to the German federal prosecutor. If they had, the police said, they would have killed a "high number" of passengers.

"We haven't had this serious a threat since 9/11," a terrorism expert, Rolf Tophoven, said. "It's clear we have people in Germany who are willing to carry out a huge and harmful attack."

While the extent of the plot is still shrouded in mystery, prosecutors said it was likely the would-be bombers were not acting alone. They may have been motivated by anger over the war in Lebanon, in which the German government has agreed to play a limited, peacekeeping role.

On Monday, prosecutors said Lebanon's military intelligence agency had offered the German authorities "decisive" information that led to the arrest of the man. That has added to worries that Germany, after years of being a fortunate bystander, is now in the sights of international terrorists.

"The threat has never been greater," the German interior minister, Wolfgang Schäuble, said in an interview over the weekend with the German broadcaster ARD.

Terrorism experts stopped short of comparing the train plot to the foiled attempt to blow up passenger planes flying from Britain to the United States. The explosives, Tophoven noted, were crudely made, and there was no evidence that the perpetrators intended to be suicide bombers.

Still, the case has punctured the sense of immunity that many Germans had because of their government's opposition to the war in Iraq, or after the recent World Cup, which went off without incident.

It is also reshaping the politically- charged debate in Germany over how far the law-enforcement authorities should be allowed to go in fighting terrorism - a debate that goes back to 2001, when it become clear that the Sept. 11 hijackers had hatched their plot in Hamburg.

On Monday, Chancellor Angela Merkel reaffirmed her support for expanded use of closed-circuit cameras in train stations and other public places. Cameras caught the images of the two men boarding trains in Cologne, and the police made the arrest a day after releasing the footage.

"We must continue to discuss the balance between video surveillance, which I'm totally in favor of, data protection, and the restriction of certain rights," Merkel said at a news conference in Berlin.

Germany, owing in part to its Nazi past, has been reluctant to pursue the more aggressive anti-terrorism measures that are standard in Britain and the United States. Berlin and other cities have fewer surveillance cameras than London, and the government does not maintain an anti-terrorism database that gathers information on all pending investigations.

Now, though, there is widening support in the political establishment for more sweeping measures.

Schäuble is pushing to install more video cameras and to create a central database with information on suspicious people in the country - something Germany has resisted, partly out of privacy concerns.

"This failed plot has brought home the reality that Germany is also a target for Islamic terrorists," said Berndt Georg Thamm, a prominent counterterrorism expert. "It just makes sense to have stronger surveillance of train stations and other places where people gather."

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Tophoven said it was inevitable that Germany would install a surveillance network like that in Britain.

"You can see the value of surveillance in this case," he said. "It's comparable to the success of the London police last summer. We had this success, fortunately, before the bombs went off."

There have even been calls for armed marshals who would ride the trains, much as sky marshals ride planes. Merkel, however, said Monday that she believed that would be going too far. Thamm said rail marshals made no sense, given the size of the German rail network.

The scope of the proposed anti-terror database also remains a divisive issue. A spokeswoman for the German data-protection commission, Ira von Wahl, said her agency would support a database that included basic information, like names, addresses and motor vehicle registrations. But a broader database would raise privacy concerns, she said, by making a wide range of personal information available to the authorities.

Such details, experts said, would be useful in unraveling this plot.

The police found the suitcases with the unexploded bombs on July 31, on regional trains in the western cities of Dortmund and Koblenz.

They considered terrorism as a factor from the outset, though there was also speculation the suitcases may have been left by a disgruntled employee of the railroad company Deutsche Bahn.

Using DNA evidence from the suitcases, as well as the surveillance footage, the police narrowed their search to Kiel, a northern university town, where the young Lebanese man was about to begin his studies. He was arrested around 4 a.m. at Kiel's central rail station.

German officials said it was probable that the suspect was part of a larger organization within Germany, though they have not yet suggested any links to Al Qaeda, Hezbollah or other groups.

"It is being investigated right now whether there are several perpetrators behind the plot," said Johannes Schmalz, president of the agency for the protection of the Constitution - a rough equivalent to the FBI - in the state of Baden-Württemberg. The German authorities, he said, had much to learn about the operation and scope of terrorist cells in Germany.

"Are these homegrown terrorists, like in London, or is it Al Qaeda?" Schmalz said. "We have to be prepared for everything. We don't have a consistent picture of the Islamic terrorists here yet."

One thing the case has done, he said, was cure Germans of any lingering belief that they are somehow immune from terrorism.

"People thought for the longest time that Germany would be safe because we didn't send troops to Iraq," Schmalz said. "This presumption is wrong. The enemy of violent Islamists is the western world as a whole."